diff --git a/src/lib/geometry.ts b/src/lib/geometry.ts
new file mode 100644
index 0000000..4131774
--- /dev/null
+++ b/src/lib/geometry.ts
@@ -0,0 +1,91 @@
+import type { Point, Shape } from './types';
+
+export type Bounds = { minX: number; minY: number; maxX: number; maxY: number };
+
+const MIN_HOLE_MM = 3;
+
+export function bounds(points: Point[]): Bounds {
+  let minX = Infinity, minY = Infinity, maxX = -Infinity, maxY = -Infinity;
+  for (const p of points) {
+    if (p.x < minX) minX = p.x;
+    if (p.y < minY) minY = p.y;
+    if (p.x > maxX) maxX = p.x;
+    if (p.y > maxY) maxY = p.y;
+  }
+  return { minX, minY, maxX, maxY };
+}
+
+export function shapeSize(shape: Shape): { width: number; height: number } {
+  const b = bounds(shape.outer);
+  return { width: b.maxX - b.minX, height: b.maxY - b.minY };
+}
+
+export function simplifyRing(ring: Point[], tolerance: number): Point[] {
+  if (ring.length <= 3) return ring.slice();
+  // Split at the vertex farthest from the start so each half is an open polyline.
+  let far = 0;
+  let farDist = -1;
+  for (let i = 1; i < ring.length; i++) {
+    const d = Math.hypot(ring[i].x - ring[0].x, ring[i].y - ring[0].y);
+    if (d > farDist) {
+      farDist = d;
+      far = i;
+    }
+  }
+  const a = douglasPeucker(ring.slice(0, far + 1), tolerance);
+  const b = douglasPeucker([...ring.slice(far), ring[0]], tolerance);
+  return [...a.slice(0, -1), ...b.slice(0, -1)];
+}
+
+function douglasPeucker(points: Point[], tolerance: number): Point[] {
+  if (points.length < 3) return points;
+  const first = points[0];
+  const last = points[points.length - 1];
+  let index = 0;
+  let maxDist = 0;
+  for (let i = 1; i < points.length - 1; i++) {
+    const d = distanceToSegment(points[i], first, last);
+    if (d > maxDist) {
+      maxDist = d;
+      index = i;
+    }
+  }
+  if (maxDist <= tolerance) return [first, last];
+  const left = douglasPeucker(points.slice(0, index + 1), tolerance);
+  const right = douglasPeucker(points.slice(index), tolerance);
+  return [...left.slice(0, -1), ...right];
+}
+
+function distanceToSegment(p: Point, a: Point, b: Point): number {
+  const dx = b.x - a.x;
+  const dy = b.y - a.y;
+  const len2 = dx * dx + dy * dy;
+  if (len2 === 0) return Math.hypot(p.x - a.x, p.y - a.y);
+  const t = Math.max(0, Math.min(1, ((p.x - a.x) * dx + (p.y - a.y) * dy) / len2));
+  return Math.hypot(p.x - (a.x + t * dx), p.y - (a.y + t * dy));
+}
+
+export function prepareForExport(shapePx: Shape, pxPerMm: number, toleranceMm = 0.5): Shape | null {
+  const toMm = (ring: Point[]) =>
+    simplifyRing(ring.map((p) => ({ x: p.x / pxPerMm, y: p.y / pxPerMm })), toleranceMm);
+  const outer = toMm(shapePx.outer);
+  if (outer.length < 3) return null;
+  const holes = shapePx.holes.map(toMm).filter((hole) => {
+    if (hole.length < 3) return false;
+    const { width, height } = shapeSize({ outer: hole, holes: [] });
+    return width >= MIN_HOLE_MM || height >= MIN_HOLE_MM;
+  });
+  const { minX, minY } = bounds(outer);
+  const shift = (ring: Point[]) => ring.map((p) => ({ x: p.x - minX, y: p.y - minY }));
+  return { outer: shift(outer), holes: holes.map(shift) };
+}
+
+export function scrapFilename(shapeMm: Shape, ext: 'dxf' | 'svg'): string {
+  const { width, height } = shapeSize(shapeMm);
+  return `scrap-${Math.round(width)}x${Math.round(height)}mm.${ext}`;
+}
+
+export function formatMm(n: number): string {
+  const rounded = Number(n.toFixed(3));
+  return String(rounded === 0 ? 0 : rounded);
+}
diff --git a/tests/geometry.test.ts b/tests/geometry.test.ts
new file mode 100644
index 0000000..75a7492
--- /dev/null
+++ b/tests/geometry.test.ts
@@ -0,0 +1,89 @@
+import { describe, expect, it } from 'vitest';
+import {
+  bounds, formatMm, prepareForExport, scrapFilename, shapeSize, simplifyRing,
+} from '../src/lib/geometry';
+import type { Point } from '../src/lib/types';
+
+/** Square ring from (x0,y0) with side `size`, one point every `step` units. */
+function denseSquare(x0: number, y0: number, size: number, step = 1): Point[] {
+  const pts: Point[] = [];
+  for (let i = 0; i < size; i += step) pts.push({ x: x0 + i, y: y0 });
+  for (let i = 0; i < size; i += step) pts.push({ x: x0 + size, y: y0 + i });
+  for (let i = 0; i < size; i += step) pts.push({ x: x0 + size - i, y: y0 + size });
+  for (let i = 0; i < size; i += step) pts.push({ x: x0, y: y0 + size - i });
+  return pts;
+}
+
+describe('bounds / shapeSize', () => {
+  it('measures the outer ring', () => {
+    const shape = { outer: denseSquare(5, 10, 20), holes: [] };
+    expect(bounds(shape.outer)).toEqual({ minX: 5, minY: 10, maxX: 25, maxY: 30 });
+    expect(shapeSize(shape)).toEqual({ width: 20, height: 20 });
+  });
+});
+
+describe('simplifyRing', () => {
+  it('reduces collinear points of a square to its 4 corners', () => {
+    const result = simplifyRing(denseSquare(0, 0, 10), 0.1);
+    expect(result).toHaveLength(4);
+    expect(bounds(result)).toEqual({ minX: 0, minY: 0, maxX: 10, maxY: 10 });
+  });
+
+  it('removes wiggles below tolerance but keeps bumps above it', () => {
+    const ring = denseSquare(0, 0, 100);
+    ring[150] = { x: 100.3, y: 50 }; // on the right edge, 0.3 out
+    expect(simplifyRing(ring, 0.5)).toHaveLength(4);
+    ring[150] = { x: 102, y: 50 }; // 2 out
+    expect(simplifyRing(ring, 0.5).some((p) => p.x === 102)).toBe(true);
+  });
+
+  it('leaves tiny rings alone', () => {
+    const tri = [{ x: 0, y: 0 }, { x: 1, y: 0 }, { x: 0, y: 1 }];
+    expect(simplifyRing(tri, 5)).toEqual(tri);
+  });
+});
+
+describe('prepareForExport', () => {
+  it('converts px to mm and moves the outer bounding box to the origin', () => {
+    const shapePx = { outer: denseSquare(100, 50, 200), holes: [denseSquare(150, 100, 40)] };
+    const mm = prepareForExport(shapePx, 4)!; // 4 px per mm
+    expect(bounds(mm.outer)).toEqual({ minX: 0, minY: 0, maxX: 50, maxY: 50 });
+    expect(mm.holes).toHaveLength(1);
+    expect(bounds(mm.holes[0])).toEqual({ minX: 12.5, minY: 12.5, maxX: 22.5, maxY: 22.5 });
+    expect(mm.outer).toHaveLength(4);
+  });
+
+  it('drops holes smaller than 3 mm in both directions (glare/speckle)', () => {
+    const shapePx = {
+      outer: denseSquare(0, 0, 400),
+      holes: [denseSquare(100, 100, 8), denseSquare(200, 200, 40)], // 2 mm and 10 mm at 4 px/mm
+    };
+    const mm = prepareForExport(shapePx, 4)!;
+    expect(mm.holes).toHaveLength(1);
+    expect(shapeSize({ outer: mm.holes[0], holes: [] }).width).toBeCloseTo(10);
+  });
+
+  it('keeps a long thin hole that is under 3 mm in only one direction', () => {
+    const slot: Point[] = [{ x: 100, y: 100 }, { x: 300, y: 100 }, { x: 300, y: 104 }, { x: 100, y: 104 }];
+    const mm = prepareForExport({ outer: denseSquare(0, 0, 400), holes: [slot] }, 4)!;
+    expect(mm.holes).toHaveLength(1);
+  });
+
+  it('returns null when the outer ring degenerates', () => {
+    expect(prepareForExport({ outer: [{ x: 0, y: 0 }, { x: 1, y: 1 }], holes: [] }, 4)).toBeNull();
+  });
+});
+
+describe('scrapFilename / formatMm', () => {
+  it('names the file after the rounded size', () => {
+    const shape = { outer: [{ x: 0, y: 0 }, { x: 412.4, y: 0 }, { x: 412.4, y: 286.6 }, { x: 0, y: 286.6 }], holes: [] };
+    expect(scrapFilename(shape, 'dxf')).toBe('scrap-412x287mm.dxf');
+    expect(scrapFilename(shape, 'svg')).toBe('scrap-412x287mm.svg');
+  });
+
+  it('formats numbers compactly', () => {
+    expect(formatMm(1.23456)).toBe('1.235');
+    expect(formatMm(2)).toBe('2');
+    expect(formatMm(-0.0001)).toBe('0');
+  });
+});